A pressure sensor is typically either a gage, differential or absolute pressure sensor. With gage sensors, pressure readings are referenced to atmospheric pressure. That is, zero output is at atmospheric pressure. A differential pressure sensor is similar to a gauge pressure sensor, but instead of measuring pressure relative to ambient atmospheric pressure, differential measurements are taken with respect to a specific reference pressure. Absolute pressure sensors sense a pressure compared to near zero pressure or a vacuum reference. That is, an output of the pressure sensor is zero at full vacuum pressure. An absolute pressure reference is commonly used in applications such as calibration, altitude simulation, and a variety of low absolute pressure regulation situations, for example.
Many pressure sensor products have a stable reference pressure that is obtained through expensive custom packages and package sealing techniques. For example, a vacuum reference for an absolute pressure sensor can be created with custom hermetic packages. FIG. 1 illustrates a typical pressure sensor 100 that includes two Wheatstone bridges with sixteen resistors on a sensing die 102. A sensing side of the die is ionically bonded to a tube 104 through which pressure is measured. The die 102 and tube 104 are then soldered into a stainless steal header 106. The sensing die 102 is wire-bonded 108 to gold plated pins 110, so that an output of the sensing die 102 can be received from the pressure sensor 100. The gold plated pins 110 are hermetically sealed within the steal header 106 with a glass seal 112. The non-sensing side of the sensing die 102 is sealed by a weld cover 114 within a vacuum. Getter 116 is placed within the weld cover to absorb helium and other gases during soldering.
The pressure sensor 100 includes hermetically sealed pins 110 and a vacuum sealed cover 114. The seals provide the reference pressure, for example, a vacuum reference, for the pressure sensor 100. Thus, a pressure measurement from the sensing die should not be affected by ambient pressure. If the pressure sensor 100 was alternatively configured as a gauge or differential pressure sensor, hermetic and vacuum seals may still be required.
Within the packaging process of pressure sensors, such as the pressure sensor 100, sealing output pins and the sensing die from ambient pressures can be the most difficult and/or time-consuming step. Thus, it would be desirable to provide a vacuum reference on the sensing die to reduce the reliance on the hermetic package and package sealing techniques to achieve the absolute pressure sensor performance.